Blackberry (former RIM) let us know a long time ago that they were going to write a custom Dalvik VM (being open source, they can do that) that could run Android apps inside the Blackberry 10 OS. It's not that different from Amazon's own store, although they are using a completely different OS than Android.
In both cases developers have to repackage their Android apps for these stores. They won't work by default. But even so, Blackberry managed to get a lot of developers to port their apps to the BB10 OS (40% of the 70,000 total, to be exact). Many reviewers have complained about this, though, because repacking apps from another OS is not exactly the best experience for the user, because the two operating systems use different UI paradigms to interact with the apps, and the experience will feel disjointed when using those Android on BB10.
Blackberry does get to brag about an impressive number of apps at launch, though – the highest a platform has ever launched with, actually. So this plays into their advantage a bit. For launch, I don't think this was too bad of a strategy. However, I do think that from now on they need to focus purely on getting developers to write native apps for the BB10 platform. If some Android developers still want to sell their Android apps in the Blackberry World store, that's fine. They should let them do it. But they shouldn't pro-actively try to do that anymore. They should try to convince as many as they can to make native BB10 apps. It's the only way to make users of BB10 feel like the apps belong there, in the long-term.
Plus, depending on another platform is not the way to succeed in the long-term, just like Mark Shuttleworth from Canonical has said when asked if they will emulate Android apps for Ubuntu phone OS. They can never know when Google will take the Dalvik VM in a completely different direction, which could give them a big headache if they will want to remain compatible with the apps written for the new stock Dalvik VM. This is why if Blackberry wants BB10 to succeed, they need to cut their ties with Android as soon as possible, and focus only on BB10 native apps.